Origin of large thermoelectric power in oxygen deficient GdBaCo2O5+x

A.A. Taskin, A.N. Lavrov, Y. Ando

Central Research Institute of Electric Power Industry, Komae, Tokyo 201-8511, Japan

The further progress in thermoelectric applications strongly relies on development of new, more efficient thermoelectric materials. This has renewed the interest in transition-metal oxides, where strong electron correlations together with the degeneracy of electronic states can bring about a large thermoelectric power. In the layered RBaCo2O5+x compound (R is rare earth), a complex interplay among the spin, charge, and orbital degrees of freedom leads to a very rich phase diagram, making this system attractive for elucidating the relation between spin-orbital states and thermoelectric properties.

We have succeeded in growing GdBaCo2O5+x single crystals by the floating-zone method and in fine-tuning their oxygen content to required values in a broad range 0<x<0.8. Here we report a detailed study of their thermoelectric as well as transport and magnetic properties.

We find that the thermopower, which is almost temperature independent at T<150K, changes its sign from negative to positive upon increasing oxygen content with a sharp divergence at x=0.5. The absolute value of thermopower reaches 800 m V/K near x=0.500 and decreases as x deviates from 0.500. The obtained experimental data suggest that in this strongly correlated transition-metal oxide the thermopower is mainly governed by the entropy contribution of charge carriers and is strongly enhanced by their spin and orbital degeneracy as has been recently proposed theoretically.